I find with Jopen the most important thing (as with most rabbits) is making sure that they fit your anatomy. I've been studying dimensions quite closely and for me the girth must be less than 4 3/4". so for me vr3 or vr4 is looking best. vr6 was WAY too girthy.
I've heard others mention distance between clitoris/vagina/g-spot .
I have a shallow g-spot and a high clit.
If you have a Jopen vanity please tell me what works or doesn't work for you and your specific body?